The present invention relates to a focusing mechanism for binoculars by which the focus is adjusted by moving the objective lenses of two telephotographic optical systems, which have optic axes thereof arranged in parallel, along the optic axis direction thereof, and more particularly to a focusing mechanism capable of improving the operating performance with a simple arrangement.
Conventionally, binoculars having two telephotographic optical systems, disposed with the optic axes thereof arranged in parallel, are generally provided with a focusing mechanism for simultaneously moving the eye pieces of right and left telephotographic optical systems in the optic axis direction thereof, and a diopter difference adjustment mechanism for independently moving one of the eye contact portions. The so-called diopter-adjusting operation is executed, as described below, by correcting a diopter difference between the eyes of an operator. The diopter difference adjustment mechanism adjusts a diopter difference between the right and left eyes of the operator and the focus adjustment mechanism adjusts the focus so that the diopter difference is adjusted.
However, in the case of binoculars when the objective lenses of two telephotographic optical systems which are disposed in parallel are fixed at the same position in the optic axes directions, a positional difference of the right and left eye contact portions (i.e., eye pieces), may be caused by a difference of the focal length of each of the objective lenses which occurs due to an error in manufacturing. In other words, the eye contact portion on the side of the objective lens having a longer focal length is more projected in the optic axis direction. In this case, the objective lens of the projected eye contact portion is forwardly dislocated by inserting a washer into the position where the objective lens is fixed, so that the right and left eye contact portions are equally positioned in the optic axes directions. Hereinafter, this adjustment is referred to as an adjustment of a difference of eye contact positions. Several kinds of washers having a different thickness are prepared and are selected and inserted in accordance to the amount of projection of the eye contact portion having been caused in accordance with a difference of the focal length of the objective lenses.
Recently proposed binoculars have been provided with telephotographic optical systems disposed in parallel with objective lenses having a uniform distance between the optic axes thereof. Wherein, the optic axis of the objective lens in each of the telephotographic optical systems is eccentrically arranged with respect to the optic axis of an eye piece by a so-called Porro prism as an erecting prism system interposed between the objective lens and the eye piece, and an eye contact portion is rotated about the axis of the objective lens thereby to adjust the width between the eyes. In this type of binocular, a focusing mechanism is provided with the objective lenses in which the distance between optic axes thereof is not changed, since it is easier to provide the focusing mechanism on the objective lenses in which the distance of the optic axes is not changed as compared with the case in which the focusing mechanism is provided on the eye contact portions arranged to be rockingly moved for adjusting the width therebetween. Thus, a focusing operation is carried out by moving the objective lenses in an optic axis direction.
Nevertheless, when a focusing operation is carried out by moving the objective lenses as described above, a problem arises in that the arrangement of the binoculars is made complex in the portion near the objective lenses. In addition, the job of adjusting a difference between the eye contact positions carried out by adjusting the position of the objective lenses, is troublesome and requires skill because attention must be paid to not make the surface of the lenses dirty by grease applied to the sliding surface of objective lens frames to smoothly move the frames of the objective lenses, when the focusing operation is carried out.
Further, the above binoculars have two kinds of adjustment mechanisms, i.e., a mechanism for adjusting a diopter difference and a mechanism for adjusting a difference of eye contact positions which are optically the same mechanism, and thus the provision of the two adjustment mechanisms is useless.